1. Field of the Invention
The invention relates to an internal combustion engine, and more particularly to an internal combustion engine having a low-pressure fuel feed system that feeds or supplies low-pressure fuel, low-pressure fuel injection devices capable of injecting the low-pressure fuel into intake ports, a high-pressure fuel feed system that feeds or supplies high-pressure fuel, and high-pressure fuel injection devices capable of injecting the high-pressure fuel into combustion chambers.
2. Description of the Related Art
As one type of internal combustion engine, such as a gasoline engine or a diesel engine, installed on a vehicle, such as a passenger automobile or a truck, a direct or in-cylinder injection type internal combustion engine in which fuel is directly injected into combustion chambers (cylinders) rather than intake ports is known in the art. In the direct injection type internal combustion engine, when an intake valve is opened, air is drawn from a corresponding intake port into a combustion chamber, and a fuel injection valve (injector) directly injects fuel into the combustion chamber during the intake stroke or during the compression stroke on which a piston is elevated so as to compress the intake air. As a result, high-pressure air and mist-like fuel are mixed with each other, and the resulting fuel-air mixture is fired by an ignition plug, to explode in the combustion chamber. Then, exhaust gas is discharged through an exhaust port when an exhaust valve is opened.
In a fuel system of the direct injection type internal combustion engine as described above, an electrically operated, low-pressure fuel pump pumps up the fuel in a fuel tank and raises the pressure of the fuel to a given low pressure, and a high-pressure fuel pump raises the pressure of the low-pressure fuel to provide high-pressure fuel. The high-pressure fuel is then stored in a delivery pipe, and a plurality of fuel injection valves (injectors) mounted on the delivery pipe inject the fuel in the form of particles into the respective combustion chambers. In the high-pressure pump, a plunger is driven by cams that rotate with the crankshaft, to move up and down or reciprocate so as to raise the pressure of the fuel. In this case, the reciprocating motion of the plunger has a suction stroke in which the plunger moves in such a direction as to increase the volume of a pressure chamber, and a feed stroke in which the plunger moves in such a direction as to reduce the volume of the pressure chamber. A flow regulating valve is provided in a fuel intake passage that communicates with the pressure chamber. The flow regulating valve is opened on the intake stroke, so that the fuel is drawn into the pressure chamber through the fuel intake passage, and the flow regulating valve is closed on the feed stroke, so that the fuel in the pressure chamber is pressurized to a given pressure and then delivered from the high-pressure fuel pump. Thus, the amount of the high-pressure fuel delivered from the high-pressure fuel pump can be adjusted by controlling the timing of opening and closing of the flow regulating valve.
In the fuel system of the direct injection type internal combustion engine as described above, the plunger is reciprocated by the cams that move along with the crankshaft. It is thus possible to appropriately regulate or control the amount of the fuel by detecting the position of the plunger that changes with the crank angle, and setting the timing of opening and closing of the flow regulating valve in accordance with the position of the plunger. When the internal combustion engine is in a relatively quiet operating condition, such as idling, however, the operating sound or noise produced upon opening and closing of the flow regulating valve becomes noticeable or annoying, and may degrade the quietness of the engine operation. In this connection, when the engine runs at idle after it has been warmed up, the fuel is relatively easily vaporized or formed into particles due to an increased coolant temperature, and the length of time from fuel injection to ignition is relatively long since the engine speed is low, while the pressure in each cylinder is relatively low. Therefore, even if the low-pressure chamber pressurized by the low-pressure fuel pump is injected into the combustion chambers, the low-pressure fuel can be sufficiently vaporized or formed into particles. Accordingly, during idling of the engine after its warm-up, the flow regulating valve is stopped at the open position, and the low-pressure fuel pressurized by the low-pressure fuel pump is passed as it is through the high-pressure fuel pump to be fed under pressure to the delivery pipe, so that the fuel injection valves inject the low-pressure fuel into the combustion chambers. In this manner, the operating sound or noise produced upon opening and closing of the flow regulating valve in the high-pressure fuel pump can be reduced or eliminated.
An example of fuel feed system of the direct injection type internal combustion engine as described above is disclosed in Japanese Patent No. 2874082.
In the fuel system of the known direct injection type internal combustion engine as described above, the plunger of the high-pressure fuel pump reciprocates in accordance with the rotation of the crankshaft. Therefore, even if the flow regulating valve is stopped at the open position and pressurizing and feeding of the fuel are stopped while the engine runs at idle after it has been warmed up, the plunger is driven all the time. Since the reciprocating motion of the plunger causes the fuel to be pushed periodically, pressure pulsation of the fuel takes place in the fuel passage, and causes variations or fluctuations in the fuel pressure in the fuel passage, and the variations in the fuel pressure are transmitted to the delivery pipe and the fuel injection valves. While a controller of the internal combustion engine controls the injection timing and injection amount of the fuel injected from the fuel injection valves, based on engine operating conditions, errors may arise in the amount of the fuel injected by the fuel injection valves if the pressure pulsation of the fuel is transmitted to the delivery pipe and the fuel injection valves. As a result, a desired or intended amount of fuel to be injected into the combustion chambers may not be injected from the fuel injection valves, and the air/fuel ratio may deviate from a target value.